In a conventional magnetic storage system, a magnetic head includes an inductive read/write transducer mounted on a slider. The magnetic head is coupled to a rotary voice coil actuator assembly by a suspension over a surface of a spinning magnetic disk.
In operation, a lift force is generated by the aerodynamic interaction between the magnetic head and the spinning magnetic disk. The lift force is opposed by equal and opposite spring forces applied by the suspension such that a predetermined fly height (or fly height) is maintained over a full radial stroke of the rotary arm actuator assembly above the surface of the spinning magnetic disk. The fly height is defined as the spacing between the surface of the spinning magnetic disk and the read/write poles of the slider.
One objective of the design of magnetic read/write heads is to obtain a very small fly height between the read/write element and the disk surface. By maintaining a fly height close to the disk, it is possible to record high frequency signals, thereby achieving a high bit density and high storage data recording capacity.
The slider design incorporates an air bearing surface to control the aerodynamic interaction between the magnetic head and the spinning magnetic disk thereunder. Air bearing surface (ABS) sliders used in disk drives typically have a leading edge and a trailing edge at which read/write sensors are deposited. Generally, the ABS surface of a slider incorporates a patterned topology by design to achieve a desired pressure distribution during flying. In effect, the pressure distribution on the ABS contributes to the flying characteristics of the slider that include fly height, pitch, and roll of the read/write head relative to the rotating magnetic disk.
In a conventional magnetic media application, a magnetic recording disk is comprised of several concentric tracks onto which magnetization bits are deposited for data recording. Each of these tracks is further divided into sectors where the digital data are registered.
As the demand for large capacity magnetic storage continues to grow, the current trend in the magnetic storage technology has been proceeding toward a high track density design of magnetic storage media. In order to maintain the industry standard interface, magnetic storage devices increasingly rely on reducing trackwidth as a means to increase the track density without significantly altering the geometry of the storage media.
A smaller track width poses several mechanical and electrical problems to the operation of magnetic disk drives. One such problem lies in its actuation and control feature, which is critical to the operation of a magnetic disk drive. In order to appreciate the magnitude of this problem, it might be important to further describe the control aspect of a conventional magnetic read/write head.
In a conventional magnetic disk drive, a read/write head features a transducer that is integrated into a slider. The slider is in turn attached to a stainless steel flexure. The flexure and the load beam to which the flexure is attached, form a suspension arm. The suspension arm is connected to one distal end of an actuator arm, which is driven by a voice coil actuator (or VCM) at the other distal end, to cause it to rotate at its mid-length about a pivot bearing.
The suspension arm exerts an elastic force to counteract the aerodynamic lift force generated by the pressure distribution on the ABS of the slider. The elastic force together with the stiffness of the suspension arm controls the stability of the actuator arm with respect to the pitch, roll, and yaw orientations. With respect to the control feature of the magnetic disk drive, during each read or write operation, there are usually two types of positioning controls: a track-seek control and a track-follow control.
A track-seek and follow control is typically commanded when data are to be retrieved from, or new data are to be written to a particular sector of a data track. Electronic circuitry incorporating an embedded feedback control software, supplies a necessary voltage to the VCM to actuate the VCM to drive the actuator arm, to which the read/write head is attached, to a target track. Thus, a track-seek control performs a low-resolution or coarse positioning of the read/write head from one data track to another data track and also following track of corresponding track pitch density
Upon the completion of a track-seek control, subsequent data operation is typically confined to within the target track. In the earlier stage of the magnetic storage technology, a typical data track is sufficiently wide so that small variations in the position of the read/write head resulting from external disturbances to the track-seek control plant do not cause the position of the read/write head to exceed the prescribed control error allowance.
As the track width reduces as a means to increase the track density and hence the storage capacity of magnetic disk drives, the foregoing single-stage actuation design encounters a significant degree of difficulty, mainly due to the excessive control error of the track-seek control using the VCM. In particular, a single-stage actuation using the VCM is found to be inadequate because the resulting control error due to external disturbances, such as inertial shock loading or noise sometimes, could cause the read/write head to be positioned over tracks that are adjacent to the target track, thus possibly causing a magnetic field disturbance of the existing data thereon.
In a worst case scenario, the data disturbances can result in a total erasure of data in the adjacent tracks after several repetitive write operations, or data corruption upon reading. Moreover, the VCM employed in a single-stage actuation is typically subjected to a mechanical resonance at the lowest natural frequency in the range of 2000 Hz –6000 Hz due to the flexibility of the actuator arm and followed by frequencies on the suspension arm in the range of 2 kHz –15 kHz.
The response of the servo-system further limits the frequency bandwidth to less than 1500 Hz. As a result, this low frequency bandwidth imposes a severe penalty on the single-stage actuation system in such a manner that the track-seek and track-follow control is unable to rapidly and precisely respond to a change in the position of the read/write head, thus causing a significant degradation in the performance of the magnetic disk drive.
To address this technical concern, it is recognized that in order to maintain the position of the read/write head in a manner that it follows a concentric path within a narrow track width of the target data track, necessary corrections to the motion of the actuator arm are required. This provision is made possible by an enhanced track-follow control, which uses a feedback on the position error signal (PES) to make an appropriate correction to the motion of the actuator arm, so as to have the position of the read/write head follow a concentric path of the target data track within a prescribed control error allowance.
Thus, in the presence of external disturbances, variations in the position of the read/write head would not cause the position of the read/write head to significantly deviate from the target position in excess of the control error allowance. To implement this track-follow control plant, a microactuator is frequently incorporated in the control feedback loop.
Various types of microactuator have been proposed, including piezoelectric (PZT) actuators, electrostatic micro-electrical mechanical systems (MEMS), and electromagnetic microactuators. By adjusting the voltage or current supplied to the microactuator, the track-follow control makes necessary corrections to the position of the actuator arm in the presence of external disturbances, so that the read/write head follows the target data track with a predetermined degree of precision.
One such design employs a rotary co-located piezoelectric actuator attached to a hinged flexure to allow the slider to rotate upon actuation. The flexure of this design generally employs two hinged islands comprising of two rectangular paddles with two offsetting hinges. The paddles are affixed by adhesives to the piezoelectric actuator, which in turn is affixed by adhesives to the slider that contains the read/write transducer.
In principle, the piezoelectric microactuator design utilizes the physical properties of the piezoelectric material to convert an elongation or contraction of the piezoelectric material under an applied voltage to impart a force couple on the hinged islands, which, in turn, induces a rotation of the stainless steel flexure and the hence slider that contains (or supports) the read/write transducer.
Under an ideal operation, the hinged islands are designed to operate in a rotation and maintain their co-planarity during operation. In practice, however, this design may not achieve its full advantages, due to an excessive deflection of the hinged islands and the flexure.
Generally, the aerodynamic lift force generated by the ABS exerts onto the two hinged islands two reaction forces of equal magnitudes. In a like manner, the suspension gram load exerts onto the flexure at the center of rotation an equivalent reaction force. This force system causes a deflection of the flexure, resulting in a vertically downward displacement of the flexure toward to piezoelectric actuator in conjunction with a vertically upward displacement of the paddles.
The combined deflection of the flexure and the hinged islands manifests itself into a number of problems. One such problem is the resulting excessive bending stresses on the hinges, which may lead to a premature mechanical fatigue of the hinges, hence the magnetic disk drive. Another such problem is the high stress build-up in the joint bonding area between an actuator and stainless steel paddles.
Another concern with this undesirable deflection is the possible contacts between the flexure and the piezoelectric actuator under loaded condition. Frequent loading contacts may cause particulation, which is undesirable for disk drive operation.
In light of the foregoing unresolved concerns, it is recognized that an improvement to the rotary co-located microactuator-based head-gimbal assembly (HGA) design is needed. This improvement should satisfactorily resolve all the concerns without adding a further complexity to the microactuator-based head-gimbal assembly (HGA) design. Preferably, this improvement should lend itself to ease of manufacturing, which would have minimal impact on the cost of production.